Gain control system



i- 1940- N. c. NORMAN GAIN CONTROLI SYSTEM Filed Sept. 23, 1937 INVENTOR N. C NORMAN ATTORNEY Patented Apr. 30, 1940 g f r a 2,199,080 V GAVIN CONTROL SYSTEM Nathaniel 0. Norman, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York,

Application September 23, 1937, Serial No. 165,321

3 Claims. (o1.179-171 1 r,

, Thisinvention relates to amplifying systems in which the gain is varied automatically to keep the output of the system within predetermined limits. T

6 Since the volume range capacityof amplifying, recording and reproducing systems is often more limited than the volume range of the original sounds it is frequently necessary or advantageous to compress the range of amplitudes of the currents representing the sounds to a range which the system can transmit without distortion. One such compression system is shown in Patent 1,922,602 granted to me, August 1933- In some systems, particularly those of high 15 power capacity, it is also desirable to provide some'means for limiting the maximum strength of signal which can be transmitted to some predetermined value. It has been proposed heretofore to accomplish this in various ways such,

for example-as by a device which decreases in impedance with increasing applied potential, connected in shunt to the line to be protected. A volume limiter of this type is shown in Patent 2,059,194 granted to D. T. Bell, November 3,

The object of this invention is to compress the volume range of the signal and limit its maximum value by means of a relatively simple, in-

characteristics. One of the grids preferably has 35 theremote cut-off characteristic of the variable-- mu pentode type tubes and the other has a sharp cut-off characteristic. With the proper bias voltages applied simultaneously to the two grids the former effects compression of the signal and the latter becomes eifective for signals of high amplitude to limit the output to the desired maximum value.

The inventionwill bemore clearly understood from the following detailed description and the 45 drawing in which: I

- Fig. 1 is an amplifying system having the volume compression and overload limiting features of the invention; and

Fig. 2 is a modified circuit in which the biases 50 on the two control grids are independently variable.

In the system shown in Fig. 1, the microphone l' or other source of signal currents is connected to the loud-speaker 2 or other receiving device 55 through the conventional amplifiers 3 and 4 of suitable gain and the volume range control-. ling stage 5, all of which are interconnected by resistance capacity coupling of the well-known type. v

The battery 6 or other power source supplies 5: potential to the voltage dividing network 1 from which the ,various operating potentials required a cathode I0, a screen grid I I, a suppressor grid 15 I2 and a plate l3, tWo' grids I4 and I5 which control the grid-plate transconductance in different ways. The grid, l4 serves as the signal grid. and has theoremote cut-off characteristic of the variable-mu pentode tubes while grid I5 20 controls the transconductance in such a Way that the gain of the tube in volts is a linear function of the bias on this grid in volts.

With'the tube working into a 100,000 ohm loa resistance and a grid bias applied simultaneously 25 to grids I4 and I5, it was found that in the region of low transconductance the gain variation with change in bias was approximately hyperbolic which indicated that linear volume compression of the desired ratio mentioned below could be obtained with a backward-acting control circuitof the type shown in Patent 1,922,- 602 referred to above. It was also found that in the region of, cut-01f due to grid !5 the decrease in transconductance with increasing negative bias, both expressed in decibels, becomes increasingly rapid. "This phenomenon suggested that overload limiting at a desired level could be obtained simultaneously with volume compression by applying a bias to grid l5 such that the tube operates on the properpoint of the grid bias-transconductance characteristic.

.In backward-acting compressors of the type under discussion, singing around the feedback path and excessive modulation of the signal by the controlvoltage is avoided by making the loss through the signaltransmission path high for, the relatively low frequency control circuit currents- The necessity for elaborate high pass filters in the signal path is avoided by keeping the gain around thefeedback path as low as is consistent with obtaining suificient control voltage without overloading the variable gain stage.

.Since these considerations limit both the ampli- 

